The leading edges of a jet plane consist mainly of the lip at the front end of the air inlet cowl of the jet engines and the slat formed by the forward edge of each wing. These leading edges are particularly exposed to the formation of ice under unfavourable temperature and humidity conditions.
The formation of ice on the lip of the air inlet cowl is liable to alter the aerodynamics in this inlet and thus disturb the passage of air towards the fan of the engine.
The formation of ice on the leading edges of the wings is also liable to adversely affect the aerodynamics of the wings by increasing their drag and reducing their lift.
As a result, it is necessary to make provision for de-icing these leading edges if need be.
One very commonplace solution is to use hot air bled from a compressor stage of the engine. This hot air is carried under pressure by the ducts of an air circulation circuit opening near the leading edge that needs to be protected against the formation of ice or that needs to be de-iced.
This solution affords many advantages because the power required can be taken directly from the engines, the air circulation circuit is lightweight, consisting essentially of hollow elements, and inexpensive. Furthermore, it is robust and reliable.
However, when using this solution for de-icing it is absolutely essential to ensure that the temperature of the air near the leading edges does not drop below the minimum temperature required for de-icing. This minimum temperature is around 200° C.
This temperature is of necessity defined on the basis of a minimum engine operating condition, at low speed, so that this minimum de-icing temperature is assured under these conditions.
This minimum engine operating condition appears in particular when the aircraft is flying at low engine speeds under what is sometimes very troublesome icing conditions, often at an altitude of between 15000 and 20000 feet for a period of the order of 45 minutes.
Modern jet engine optimization means that when these engines are developing their maximum power, the temperature of the air bled from a compressor stage of the engine may, in an engine maximum operation condition at high speed, be as high as 400° C. or even 600° C.
Now, a temperature as high as this is incompatible with the composite materials increasingly used in an attempt to reduce the weight of the aircraft. Nowadays, over 50% of the elements of a commercial airliner are made of these composite materials. The consequence of this is that elements made of composite material can sometimes be found at points which, at least intermittently, can be exposed to relatively high temperatures.
For this reason, it is absolutely essential to protect some of the elements made of composite material against heat. This entails the addition of protective elements (refer for example to Patent Application FR 2823533) which therefore represent an undesirable additional weight and make the structure more complicated.